This question on Physics.SE asks whether other colors are possible for the sky.

I would like my planet to be inhabitable by normal humans. It has 768 days and they are living in a temperate/Mediterranean climate area. I am not yet sure what color the sun is, but I'd like the sky to be near blue, but more violet. Would that change the color of grass or leaves? I understand that we have trees with color variation on Earth, but this is an overview. A child would say leaves of summer are green. I think that as my sky is just slightly off color, that trees would be the same, slightly a different green.

$\begingroup$My hub is saying that the planet should be as Earth-like as possible because having had 97000 years to find it from their seed ship, the AIs would have found one as Earthlike as possible. I maybe making this too difficult. AND it is not necessary to the story. It's just background.$\endgroup$
– WRXDec 3 '16 at 17:00

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$\begingroup$You really aren't going to get a violet sky unless you significantly change the atmospheric composition (and perhaps not even then - see Rayleigh scattering), in which case you won't have a very Earthlike planet. About the best you could realistically do is the deep blue sky of altitudes over ~12K ft/4Km on Earth. But that doesn't affect leaf color much.$\endgroup$
– jamesqfDec 3 '16 at 18:13

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$\begingroup$I remember reading this article a few years ago, and it may have information that would be very useful! Unfortunately, the issue it's from is behind an 8 dollar paywall.$\endgroup$
– JNWDec 3 '16 at 18:37

$\begingroup$Thanks, 8 bucks is a lot when everyone here is giving me great advice for free and it isn't critical. I appreciate it and may head down to the local library this week.$\endgroup$
– WRXDec 3 '16 at 19:28

3 Answers
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Reasons for plant coloration

Plants would develop pigments that let them absorb the parts of the spectrum that are most valuable to them. Here are the absorption spectra for the two types of chlorphyll in plants:

As you can see, plants absorb almost nothing in the green range, so green light is reflected; thus plants look green.

Red algae, on the other hand, live deep underwater where there is only blue and violet light; shorter wavelength light penetrates water more deeply. Thus, red algae have absorption spectra that look like this:

The carotenoids give them significantly enhance absorbance in the blue and green regions, so that they reflect more yellow to red light giving them a red appearance:

Your sky is slightly shifted to the violet so there is more violet light available relative to the more blue light available on earth. There should still be plenty of red light available because you aren't under the ocean.

Conclusions

I think there are two possible explanations for your flora. Either the primary absorption pigments (chlorphyll) are shifted to the violet; this means the wavelength gap would be more blue-green than green.

Alternately, you could say that all your plants have a secondary pigment (like caroteniods) that give your plant more absorption in the violet-blue range. This will make your plants appear more green-yellow.

$\begingroup$thank you so much! My science fiction story is fiction but I don't want someone with a clue (unlike me) to say the plants can't be whatever colour. My hub suggested we use a violet light on a green plant and decide. That is also a good idea.$\endgroup$
– WRXDec 3 '16 at 16:48

$\begingroup$@kingledion: "plants absorb almost nothing in the green range, so green light is reflected" . There is no correlation. No absorption would mean usually transmission (and not necessarily reflection). Reflection is due as protection (alpine plants tend to have also much more white reflection, or also blue), to protect from stronger sun.$\endgroup$
– Giacomo CatenazziDec 3 '16 at 20:22

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$\begingroup$@GiacomoCatenazzi See answer 2 in this link. I don't know what you would mean by transmission. If a blue or red light photon hits a plant, it is more likely to be absorbed. If a green photon hits a plant, it is more likely to be reflected. Thus when you point your eyes at a plant, green photons are more likely to be reflected into your eyes; plants appear green. Also, white is a mix of all colors. If a plant reflects white light then it is reflecting the complete visible spectra.$\endgroup$
– kingledionDec 3 '16 at 20:29

$\begingroup$@kingledion: the third kind of interaction of light with matter: light pass thru the matter.$\endgroup$
– Giacomo CatenazziDec 3 '16 at 20:34

Color of foliage is based on whatever the color is of bacteria that get incorporated to become chloroplasts.Or more specifically the color of their light absorbing pigments. there is a huge range in nature for color in photosynthetic organisms, plants are green becasue chlorophyll is green, it could have just as easily been red or purple. http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html

the only rule is if there is a wavelength of light not making it through the sky you don't want that color. heck chlorophyll reflects the most abundant wavelengths from our sun. there is decent evidence that chloroplast ancestors absorb the margins of the visible spectrum becasue halobacterium absorb the major constituents, becasue the chropyll users could not compete with them directly.

$\begingroup$It's important to realise that we don't have green plants because that's most efficient: most efficient would actually be black plants that can absorb all wavelengths. But evolution hasn't favoured that primarily because it hasn't needed to. Plants could have evolved with just about any spectrum, they just happened to have evolved with this one, probably because by absorbing two different spectra it allowed them to outcompete an earlier branch of life that only absorbed green light (and hence had purple pigment, like the halobacteria mentioned).$\endgroup$
– Periata BreattaDec 5 '16 at 7:33

$\begingroup$actually it looks like the halobacteria actually were winning they absorb a much broader band of the spectrum, chlorophyll absorbs the colors halobacteria don't as if they evolved to not compete with it. Green plants are thought to have one for unrelated reasons like symbiosis or changes in ocean salinity.$\endgroup$
– JohnDec 5 '16 at 15:08

The light of sun is white: if you put a white sheet in lawn, you see it white, not the blue/azure like sky. The colour of sky is due to soft scattering of light, but direct light has very few scattering, so white.

BTW because of scattering, light "loses" few blue, so the light is "less blue" as original light (e.g. as seen on space or on the moon), and not more blue as it seems by the question.

Additionally, our eye has chromatic adaption, which "correct" most of colours (note: but mostly in direction of red, not really much adaption in direction of blue/violet), so we tend see (in brain) the colour of object, not the colour of the light of the object (as seen from the eyes).

So the color of leaves are correlated on color of the sun, not really about the color of the sky, but if the sky is really darker (so if the sky absorb light).

So, with a sun like our but a planet with light violet sky, I would assume leaves would still be green.

Note: You should carefully choose what it is "violet". If it go in direction of purple, it means scattering of blue and red, which means (in case of darker sky) that the most important light reaching ground is predominately green, so plants would not discard it.

$\begingroup$whose assumption? Sorry, I am not sure what you mean. I do appreciate your answer. Oh and I vote for every polite answer because I like it that people are taking the time to give such thoughtful answers.$\endgroup$
– WRXDec 4 '16 at 15:58

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$\begingroup$@WillowRex: we confused that colour of sky will be the same as colour incident to leaves. You question what A if B? so we looked B before realising that the two are nearly independent.$\endgroup$
– Giacomo CatenazziDec 4 '16 at 16:19

$\begingroup$thanks. My problem is that my science and math knowledge are minimal, so I often do not know how to ask my questions properly.$\endgroup$
– WRXDec 4 '16 at 16:22